Frederick Jackson Turner, the great historian of the late 19th century, said it was on the frontier that democracy was born, that American ideas of equality were born, individualism. But the frontier also carried with it the expansion of slavery. The westward expansion of slavery was one of the most dynamic economic and social processes going on in this country. The westward expansion carried slavery down into the Southwest, into Mississippi, Alabama, crossing the Mississippi River into Louisiana. Finally, by the 1840's, it was pouring into Texas. So the expansion of slavery, which became the major political question of the 1850's, was not just a political issue. It was a fact of life that every American had experienced during this period.

- Eric Foner, historian

In 1828, when Andrew Jackson was elected president, he promised to expand the United States westward. His justification for expansion seemed to him self-evident, for "What good man would prefer a country covered with forest and ranged by a few thousand savages to our extensive republic, studded with cities, towns and prosperous farms and filled with all the blessings of liberty, civilization and religion?" The idea of Manifest Destiny, which held that America had the right and even the duty to extend its civilization, fueled the westward move into Indian and Mexican territories. As the country expanded, the question of slavery gained new importance. Would the new territories be slave-holding or free?

Over the next several years, Jackson seized millions of acres of Indian lands in the southeast, eventually removing five Indian nations from their homes. As a result of the removals, these lands were opened to settlement and the establishment of new cotton plantations. Although some Indian nations chose to fight, in the end, most of the population was removed. The U.S. Army forcibly removed more than 15,000 Cherokees and marched them westward to "Indian Territory" in Oklahoma, a journey the Cherokee called the "Trail of Tears." As many as a quarter of the Indians died during the forced march, and the $6 million cost of the removal was deducted from the $9 million offered them for their land in the treaty they were forced to sign. The Removal Act of 1830 guaranteed the Indians lands in the west, but these promises were later broken.

When Mexico encouraged settlers to come to its territory of Texas, many Americans brought slaves with them. In 1829, the Mexican government abolished slavery in Texas to try to lessen American influence. American slaveowners forced their newly-freed slaves to sign life indenture contracts, and the American population continued to grow even after Mexico forbade further immigration in 1830. Tensions mounted, and in 1835 a rebellion broke out. Although the Mexican army won initial engagements at the Alamo in San Antonio and the fortress of Goliad, a surprise attack by the Americans at the San Jacinto River in April, 1836 gained Texas its independence. Texas requested immediate admission into the United States, but opposition from the North to the addition of another slave state to the Union, then precariously balanced with 13 proslavery and 13 antislavery states, delayed its annexation until 1845. The annexation of Texas soon led to the Mexican War, lasting from 1846 to 1848.

Mexico severed diplomatic ties with the U.S. in protest after the annexation of Texas. Newly-elected President James K. Polk, rather than retreating, demanded even more concessions. In addition to extending Texas lands 150 miles southward to the Rio Grande, he demanded that Mexico cede New Mexico and California. The resulting war, presented by Polk as an invasion by Mexico of American land, divided the country, with Democrats siding with expansion, and Abraham Lincoln calling the war a "conquest brought into existence to catch votes."

Among the territories gained by the United States was California, where gold was discovered in 1848. The California Gold Rush swept the nation, sending almost 200,000 prospectors to California over the next four years. As the population swelled, California applied for statehood, and the compromise that allowed the state to be admitted as a free state included passage of the Fugitive Slave Law.

One northerner wrote in response to the Kansas-Nebraska Act, "We went to bed one night, old-fashioned, conservative, compromise, Union Whigs, and waked up stark mad Abolitionists."

The Kansas-Nebraska act in 1854 brought antislavery and proslavery proponents head-to-head in a battle over the status of Kansas. Slavery had been prohibited in the Great Plains territories under the Missouri Compromise of 1820. With the Kansas-Nebraska Act, the Nebraska territory was divided into north and south, and the question of whether slavery would be legal in either part was left to popular referendum.

The southern part, Kansas, soon became a battleground. Free-Soil settlers were recruited from the northeast, while secret societies on the Missouri border vowed to combat these "negro thieves." Most of the northern settlers were not abolitionists, but members of the Free Soil movement, a group of homesteaders who wanted to keep slaveholders and blacks, whether free or enslaved, from competing with them for land. The clash between proslavery and antislavery forces led to a series of violent outbreaks that historians have called Bleeding Kansas, a preview of the Civil War. More than 50 men died before Kansas declared itself a free state.

The Kansas-Nebraska Act led to the birth of the Republican Party, which promoted an anti-slavery interpretation of the Constitution. Abolitionists found a home within this larger political organization that, while not abolitionist, was against the spread of slavery.

The debate over free versus slave territories reached a new pitch with the case of Dred Scott. In 1847 Dred Scott sued for his freedom on the grounds that his master had brought him to live in free land. Ten years later his case was decided by the Supreme Court, which handed down the infamous decision that Scott could not sue because he was not a citizen of the United States and that no one of African origin could ever become a citizen.

Chief Justice Roger B. Taney justified his decision by insisting that the Founders believed that African Americans were "beings of an inferior order, and altogether unfit to associate with the white race, either in social or political relations; and so far inferior that they had no rights which the white man was bound to respect." The controversial decision invalidated the idea of a free state, since any slave brought North by his or her owner would remain trapped in a legal state of bondage. Abraham Lincoln used the Dred Scott case to launch his bid for the U.S. Senate on June 16, 1858.